Electric motors often include mechanisms that terminate operation of the motor in response to thermal overload conditions that could result in permanent damage to the motor or associated equipment. A thermal overload, such as an excessively high winding or rotor temperature, may occur as a result of a locked rotor, a high mechanical load, a supply overvoltage, a high ambient temperature, or some combination of these conditions.
Conventional thermal overload protection mechanisms are based on a thermally responsive switch or relay that interrupts the flow of electrical power to the motor when the temperature of the motor winding or other motor component reaches a predetermined elevated temperature. One typical approach uses a bimetallic switching element that is thermally coupled to the motor. The bimetallic element may comprise one of a pair of electrical contacts so that electrical currents supplied to the motor are conducted directly through the element. Alternatively, the bimetallic element may actuate or control an auxiliary pair of electrical contacts that carry power to the motor. In either event, under thermal overload conditions, the bimetallic element interrupts the electrical connection supplying power to the motor.
Another known approach uses shape memory alloy beams or springs to disengage the motor brushes from the commutator. In this approach, a shape memory alloy element returns to a "memorized" or undistorted condition in response to a high temperature condition. This reversible memory effect is used to lift the brushes away from the commutator to interrupt operation of the motor in response to a thermal overload, and returns the brushes into contact with the commutator once the temperature within the motor has fallen below a predetermined threshold.
The known approaches described above have several significant drawbacks. First, these approaches are all designed to provide multiple switching events and are self-resetting. Thus, a thermal overload condition resulting from a locked rotor condition, for example, may cause continuous limit cycling around the thermal overload condition as long as the locked rotor condition persists. Such repetitive cycles up to the thermal limit of the motor may cause damage or substantially reduce the useful operating life thereof. Second, the self-resetting switches described above may become less reliable in providing protection due to wear and fatigue caused by repetitive flexing. Finally, the self-resetting approaches discussed above are complex, consume a significant amount of space within a motor, are difficult to assemble, and are costly.